Vacuum cleaners have the disadvantage that filters will get clogged after some time of use. This phenomenon is hard to perceive by users, because the pollution of the filters is a slow process. In normal use it can take more than half a year before the performance of the vacuum cleaner is impaired.
Some vacuum cleaners are equipped with a pressure switch, measuring the pressure difference over a filter. The disadvantage of this principle is that, at varying flow, the pressure difference does not relate directly to the filter pollution. When lowering the flow by changing the setting of the speed regulator, the pressure difference will decrease and thus the signal generated by the pressure switch might change from ‘polluted’ to ‘clean’, whereas the filter pollution has not changed. There will also be a substantial difference in flow if the vacuum cleaner is used on carpet compared to the use on hard floors.
JP2008301878 provides a vacuum cleaner capable of detecting the clogging of a filter through which air from a dust collection chamber passes toward an electric fan. The dust sucked by the operation of the electric fan is stored inside a dust cup, and air from the dust cup passes through the filter toward the electric fan. The amount of the dust stored in the dust cup is detected by a dust sensor, and the air flow rate of the electric fan is detected by an air flow rate detection part based on the change of the electric current in the electric fan. When the result of detection by the dust sensor suggests that the amount of the dust is not more than a prescribed quantity and when the result of detection by the air flow rate detection part suggests that the air flow rate is not more than a prescribed small air flow rate, a filter clogging determining part determines that the filter is clogged.
U.S. Pat. No. 4,294,595 discloses an automatic shutoff arrangement for a vacuum cleaner of the “clean air” type in which the vacuum fan is downstream of the dirty air passageway and vacuum filter. A pressure differential or air flow responsive switch is connected between the nozzle inlet to the dirty air passageway of the vacuum and the clean air passageway after such air flow has passed through the filter bag. The pressure differential switch is operable in response to a change in the flow of air through the system and causes power to the vacuum motor to be interrupted and simultaneously gives a warning to the operator that the filter is full or that a blockage exists somewhere along substantially the entire length of the dirty air passageway.
U.S. Pat. No. 5,294,872 discloses a vacuum cleaner in which by detecting a rotational speed of a variable speed fan motor adapted to give a suction force to the cleaner and its change range, the choking state of the filter and the state of the cleaned surface are discriminated, and a speed command of the fan motor is corrected on the basis of the result of the discrimination, and comfortable cleaning can be performed by the optimum suction force.
Prior art filter monitoring systems do not effectively cope with flow variation and the power setting. This will lead to systems that signal a polluted filter correctly and after lowering the power will show a non-polluted filter again, whereas the filter is still as polluted as it was. On the other hand these appliances will show a polluted filter too early at high flows, e.g. when the nozzle is not on the floor.